1. Field of the Invention
The present invention relates to a digital X-ray image read apparatus, more particularly, it relates to a digital X-ray image read apparatus with a correction function for correcting an X-ray image after A/D conversion. A digital X-ray image read apparatus according to the present invention is mainly used in the field of medical equipment.
2. Description of the Related Art
An X-ray apparatus is widely used in various fields, particularly in the medical field. As a conventional X-ray apparatus having high sensitivity and high resolution, there is a digital X-ray image read apparatus that displays an X-ray image on a cathode-ray tube (CRT) instead of a radiograph.
The above conventional digital X-ray image read apparatus (see, for example, U.S. Pat. No. 3,859,527) uses a photostimulable phosphor plate constituted by a sheet-like fluorescence medium which is able to accumulate a part of the X-ray energy. This fluorescence medium accumulating the X-ray energy is called an "accumulative fluorescence body". The accumulative fluorescence body can accumulate X-ray energy for a relatively long time.
In general, a digital X-ray image can be obtained by the following steps. That is, when X-rays are irradiated onto an object (for example, a human body), the X-rays transmitted through the object expose the photostimulable phosphor plate. When the photostimulable phosphor plate is scanned by an excitation beam (for example, a laser beam), the energy accumulated on the fluorescence body is excited by the laser beam and a fluorescent light corresponding to the accumulated energy is emitted from the photostimulable phosphor plate.
The fluorescent light is collected by collection equipment, for example, bundled optical fibers, and converted to analog electrical signals by an optical-to-electrical converter. Further, the analog electrical signals are converted to digital signals to provide a digital X-ray image on the CRT.
Next, a doctor reads the X-ray image displayed on the CRT. This read operation by the doctor is called an "actual read". That is, the actual read, in this case, means that the doctor reads the X-ray image displayed on the monitor image when diagnosing the object. For the photographic conditions during the actual read, it is necessary to determine a supply voltage for an X-ray tube, a multiplication rate of a photomultiplier, an amplification rate of the amplifier, and a distance between the X-ray tube and the object.
There is, however, a problem in the above conventional system. This problem is the fact that the actual read operation must be performed many times when diagnosing the object. This problem is caused by an afterglow of the fluorescent light from the photostimulable phosphor plate. That is, the afterglow of the fluorescent light occurs for a long time just after the irradiation of the excitation beam is stopped.
In this case, since the amount of the fluorescent light at the irradiation of a subsequent excitation beam is defined by the sum of the fluorescent light at that time and the afterglow occurring in previous steps. Accordingly, it is difficult to determine the precise amount of fluorescent light at that time.
Accordingly, it is necessary to wait for a relatively long time until all afterglow attenuates to a predetermined light level.